Service leaders grounded the stealthy twin-engine fighter May 3, after 14 incidents when F-22 pilots suffered “hypoxia-like symptoms.”
Air Force officials initially suspected a problem with the aircraft’s On-Board Oxygen Generation System (OBOGS), but that is looking less likely, the sources said.
Instead, investigators now suspect that carbon monoxide generated by the plane’s jet engines is getting into the cockpit.
Part of the problem may be the procedures used at Elmendorf Air Force Base, Alaska, where most of the known incidents have occurred. Because of the harsh climate, pilots often start their jet engines inside a hangar before taking off. Investigators suspect that exhaust gases are getting trapped in the building and subsequently sucked back into the engines, where they enter the bleed air intakes that supply the OBOGS, sources said.
The design and placement of the intakes, which are located within the engines’ compressor sections, are fairly standard for jet aircraft.
There is no immediate fix in sight, sources said.
Asked for comment, an Air Force spokesman said he had no further information at this time.
“The safety of our aircrews is paramount, and the Air Force continues to carefully study all factors of F-22 flight safety,” said Maj. Chad Steffey.
One aviation safety expert said that if the hypoxia is being caused by carbon monoxide in the cockpit, the gas is likely being generated by the plane’s engines.
“I would think that it has something to do with exhaust flow somehow getting into the oxygen generating system,” said Hans Weber, who sat on the U.S. Federal Aviation Administration’s Research, Engineering and Development Advisory Committee, and is president of Tecop International, a San Diego consulting firm.
Even a small amount of the colorless, odorless gas can have serious effects, Weber said.
“It doesn’t take a large concentration of carbon monoxide to start affecting people, making them ill — and not just ill but really diminishing their ability to perceive anything.”
Weber said the difficulty of the fix will depend on the problem.
If the carbon monoxide is being ingested because the engines are being started in confined spaces, a fix could be as simple as moving the jet outside, Weber said. If the engine must be started inside the hangar, the startup of the oxygen system might be delayed until the jet is out in the open, he said.
But if dangerous levels of carbon monoxide are entering the cockpit despite these changes, the Air Force might have to add bulky cartridges or scrubbers to the life-support system, he said.
The Navy has had similar problems with the OBOGS on its F/A-18 Hornet, which sucked carbon monoxide into its oxygen system during carrier operations.
Between 2002 and 2009, Hornet aviators suffered 64 reported episodes of hypoxia, including two that killed the pilots, according to the July-August 2010 issue of “Approach,” a Navy Safety Center publication.
Some 77 percent of the incidents happened in single-seat Hornets, which saw 3.2 incidents per 100,000 flight hours. The two-seat version saw 1.7 incidents per 100,000 flight hours.
According to Navy documents, “Prolonged exposure to jet engine exhaust while sitting behind another aircraft waiting to take off and operating with low bleed air pressures can result in carbon monoxide (CO) breaking through … into the pilot’s breathing gas.”
The Navy modified the planes’ OBOGS to fix the problem, has had no recent similar incidents, and is not currently investigating the systems, Naval Air Systems Command officials said.
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